In recent years, home theaters where one can enjoy a highly realistic feeling of a movie theater within the home, are becoming popular. A home theater with multiple speakers installed to surround the listener as represented by a 5.1-channel surround is common. A surround system realized by multiple speakers in this way, however, required complex wiring systems to each speaker, and also had the problem that space was required to install multiple speakers.
Audio playback systems are being proposed (for example, refer to the Japanese Unexamined Patent Application, First Publication No. 2005-64746) using a speaker array with a plurality of speaker units disposed in lines that create virtual sound sources surrounding the listener making use of reflections of the audio beam of the speaker array from the wall faces of a room.
FIG. 7 shows the construction of the line array speaker in the audio playback system described in the Japanese Unexamined Patent Application, First Publication No. 2005-64746. This line array speaker is composed of a plurality of speaker units 21 (21-1 to 21-n) in slender cases disposed side by side on a line. Each speaker unit 21 is disposed at equal intervals with a spacing d; the width of the speaker array is L.
If an audio signal of the same phase is input to a plurality of speaker units 21, the synthesized wavefront of audio output from all the speaker units 21 become parallel audio beams that propagate only toward the front. Audio components that propagate in directions other than the front are canceled out (by mutual interference) when the components output from each speaker unit 21 are synthesized, and only the components directed toward the front are reinforced by synthesis and remain as audio beams. When audio output from the speaker unit 21 is sequentially delayed from one end to the other end, the synthesized wavefront inclines according to this delay time so that the audio beam can be directed in an inclined direction.
In this way, by controlling the delay quantity of the audio signals input to a plurality of speaker units, the audio beam can be directed in the target direction (directional characteristics can be controlled).
If the speaker array width L is increased (the number of speaker units increased) in the line array speaker shown in FIG. 7, the directional characteristics become sharper, and the audio beam can be concentrated in the target direction. Moreover, if the speaker array width is increased, direction control is possible even on the low frequency band side.
The beam width of the audio beam is determined by formula 1 given below (wherein v is the velocity of sound, f is the frequency).θ=sin−1 (v/fdn)  Formula 1
To increase the speaker array width, the number of speaker units may be increased; alternatively, to increase the speaker array width with the same number of speaker units, the spacing d may be increased. If the speaker unit spacing d is increased, however, the problem of audio beam generated in a direction other than the target direction may occur because of the spatial alias, so direction control in the high frequency band becomes difficult. To ensure that a different audio beam is not generated, d should be set such that the conditions in the formula 2 below are satisfied.d<v/2f  Formula 2
For example, when the spacing d of the speaker units is 4.5 cm (d=4.5 cm), width L of the speaker array is 67.5 cm (L=67.5 cm), then from Formula 1, the low frequency side of the frequency band at which direction is controllable is about 500 Hz, and from Formula 2, the high frequency side becomes 4 kHz approximately. Accordingly, the frequency band at which direction was controllable was 500 Hz approximately to 4 kHz approximately. Playback of bandwidth used for telephone voice was possible, but playback of bandwidth required for home theaters (for example, 250 Hz to 12 kHz approximately) could not be realized. To realize this, the number of speaker units needs to be increased, but the problem that arises is that cost increases when the number of speaker units increases.
In this way, a trade-off relationship exists between the improvement in direction controllable frequency and the suppression of cost.
Consequently, an array speaker system that enables arbitrary design of direction controllable frequency band according to the required frequency band is demanded.
In teleconferences and the like, the narrator's voice is required to be picked up correctly by the microphone. For this reason, a directional microphone is used and sound in the direction of the narrator is efficiently picked up.
Additionally, a pickup apparatus for directivity control has been proposed (for example, refer to Japanese Unexamined Patent Application, First Publication No. 1993-91588) using an array microphone (line) composed of a plurality of microphone units, and setting the delay time in the output of each microphone unit.
FIG. 14 shows the construction of a line array microphone. This line array microphone is composed of a plurality of microphone units 221 (221-1 to 221-n) in slender cases disposed side by side on a line. Each microphone unit 221 is disposed at equal intervals at a spacing d2, and the width of the array microphone is L2.
Plane sound waves (sound waves at the same phase) that reach a plurality of microphone units 221 normally from the front side are picked up by each microphone unit 221. When the audio signals output by each of the microphone units 221 are synthesized, they are reinforced because they are in the same phase. On the other hand, sound waves that arrive from a direction other than the front side (for example, from the side of the line array microphone), differ in phase from the audio signals output by each of the microphone units 221; thus, when synthesized, they cancel out each other. Accordingly, the sensitivity of the array microphone is reduced in beam form, and the main sensitivity (main beam) is formed only in the front direction.
Here, if the audio signal from each microphone unit 221 is sequentially delayed from one end to the other end, the pickup direction at which the maximum level occurs inclines according to the delay time, and the main beam can be directed in an inclined direction.
In this way, by controlling the delay quantity in the audio signal output from a plurality of microphone units, sound can be picked up from the target direction (directional characteristics can be controlled).
If the width L2 of the array microphone is increased (the number of microphone units increased) in the line array microphone as shown in FIG. 14, the directional characteristics become sharper, and the main beam can be concentrated in the target direction. Additionally, if the width L2 of the array microphone is increased, direction control is possible on the side of lower frequency bands.
The beam width of the main beam is determined by Formula 3 given below (wherein v is the velocity of sound, f is the frequency).θ=sin−1 (v/fd2n)  Formula 3
To increase the width L of the microphone array, the number of microphone units may be increased, or the microphone unit spacing d2 may be increased keeping the number of units the same. If the microphone unit spacing d2 is increased, however, the problem that may occur is that the main beam is generated in a direction other than the desired direction because of the spatial alias, so direction control in the high frequency band becomes difficult. To ensure that a different main beam is not generated, d2 should be set such that the conditions in the Formula 4 below are satisfied.d2<v/2f  Formula 4
For example, if the microphone unit spacing d2=4.5 cm, and the microphone array width L2=67.5 cm, the low frequency side of the frequency band at which direction is controllable in the range of beam widths 3 dB below the peak value becomes 500 Hz approximately according to Formula 3 to arrive at a value of θ±30°, and becomes approximately 4 kHz on the high frequency side according to Formula 4. Thus, the frequency band at which direction is controllable became approximately 500 Hz to approximately 4 kHz, and the bandwidth pickup of telephone voice approximately was realized; however, bandwidth pickup (for example, 250 Hz to 12 kHz approximately) required for music recording applications could not be realized. To realize this, the number of microphone units needs to be increased, but the problem that arises is that the cost increases when the number of microphone units increases.
In this way, a trade-off relationship exists between the improvement in frequency at which direction control is possible and the suppression of cost.
Consequently, an array microphone system that enables arbitrary design of direction controllable frequency band according to the required frequency band is demanded.